The present invention relates to solid-state flat-panel display devices and more particularly to such display devices having means to optimize the luminance of pixels in the display through the use of optical feedback information from the pixels.
Solid-state organic light emitting diodes (OLEDs) are of great interest for use in flat-panel digital display devices. These display devices utilize current passing through thin films of organic material to generate light. OLED materials are also responsive to electromagnetic radiation and, when appropriately biased within an electrical circuit, can produce a current dependent on the ambient light. For example, U.S. Pat. No. 5,929,845, issued Jul. 27, 1999 to Wei et al., describes a system that both emits and detects light using an organic electro-luminescent apparatus.
The luminous efficiency of the OLED devices often decreases significantly with operation due to the instability of the organic materials. This instability may be as a result of aging, usage, temperature changes, humidity, or other environmental stresses. Light output can vary from pixel to pixel due to processing variations, as well, causing display uniformity problems.
Attempts have been made in the prior art to optimize particular display systems to overcome some of the problems noted above. For example, U.S. Pat. No. 5,216,504 issued Jun. 1, 1993 to Webb et al. describes a digital control device within a video monitor to calibrate or otherwise optimize the display, either with human input or under automated computer control.
Some systems integrate user-controlled mechanisms to provide more flexible operation or optimal use under varying conditions. For example, brightness and contrast controls are often available on CRT and LCD display devices. These controls can be based on information from the device itself, using a reference pixel within the display. U.S. Pat. No. 5,157,525; issued Oct. 20, 1992 to Eaton et al. describes the use of a reference pixel with separate control to maintain a pre-selected value for contrast or absolute brightness using a feedback arrangement which includes an LCD reference element. The feedback information is determined by measuring the average transmissivity of the LCD material with a photo-detector.
U.S. Pat. No. 5,910,792 issued Jun. 8, 1999 to Hansen et al. compares current passing through a resistive layer with that of a current source to provide a feedback compensation for temperature-induced brightness variation. It is also known to calibrate display devices through the use of external sensors which measure the light output from the display device and construct a calibration table for use by the device. See for example U.S. Pat. No. 5,371,537, issued Dec. 6, 1994 to Bohan et al. This approach has the problem that the sensor device obscures the display during the calibration and is not capable of providing real time operation. Another problem with these approaches is that the feedback does not directly respond to the emissivity of the pixels themselves, or address problems with different types (e.g. colors) of pixels within a display. Moreover, these approaches are not useful for correcting uniformity variations among individual pixel display elements.
There is a need therefore for an improved addressable display pixel design providing optical feedback that avoids the problems noted above.
The need is met according to the present invention by providing an image display that includes an addressable image display pixel, having a substrate; a light emitter formed on the substrate; a photo-sensor formed on the substrate and optically coupled to the light emitter to detect light emitted by the light emitter to generate a feedback voltage signal in response to light emitted by the light emitter; and, a feedback readout circuit formed on the substrate and responsive to the feedback voltage signal to provide a feedback signal representing the light output of the light emitter, the feedback readout circuit including a transistor amplifier, means for resetting the readout circuit, and a select switch.
The advantages of this invention are the ability to correct for non-uniformity and the ability to correct for changes due to aging in emissive digital image display devices.